The field of the invention relates generally to gas turbine engines and, more particularly, to a method and systems for automatically controlling the thrust output of a gas turbine engine to compensate for engine deterioration that may occur over time.
In at least some known jet engines, manufacturing and assembly tolerances produce variations in the flow and efficiency characteristics of the major engine components and produce variations in pressure drops and parasitic flows through the engine. These variations produce significant variations in the engine's output thrust and in its operating temperatures. Additional changes in engine thrust and operating temperatures may occur over the operating life of the engine as engine components deteriorate during in-service use. These variations are accommodated in the design process by selecting a primary control mode which is least sensitive to the expected component variations and incorporating sufficient temperature margin into the design process so that a fully deteriorated engine can be operated at required thrust levels.
The engine red-line temperature is determined from the average engine temperatures and the incorporated temperature margin. Engines which reach or exceed red-line temperature are removed from service for repair or refurbishment. Turbine cooling requirements are determined from the red-line temperature. Because minimum thrust engines operate with relatively cooler temperatures and maximum thrust engines operate relatively higher in temperature, reducing operating temperatures may be accomplished without affecting the thrust of the minimum thrust engines.
Tolerances in the design and manufacture of gas turbine engines and control system variations will create variations in operating characteristics from engine to engine within an engine model. The engines are operated according to predetermined engine model control schedules which are derived from average engine component characteristics. Un-deteriorated engines or engines with high quality components will generally run cooler and produce less thrust than average. Excess thrust levels, with accompanying higher operating temperatures, will result from lower quality components and increasing deterioration. Over time, continued operation at higher than average operating temperatures may cause components to deteriorate more rapidly and decrease the operating life of the engine.
At least some known gas turbine engines use a control architecture that is designed to enable the ‘minimum’ or ‘worst case’ engine to produce the required thrust at the target thrust setting parameter (such as fan speed). These schedules are derived from average engine performance and are then biased to meet, or exceed, required levels of thrust for all expected levels of component quality or deterioration.
It has not been possible to tailor the nominal control schedules on an engine-by-engine basis in a cost effective or reliable manner. As a result, the use of a thrust setting schedule based on the ‘minimum’ or “worst case” engine performance generally produces a large population of engines with excess thrust.